This invention relates to the field of carbon monoxide detectors and control mechanisms associated therewith.
Apparatus and methods to detect the presence of carbon monoxide in the atmosphere have been developed and are known to the prior art. However, since carbon monoxide is odorless, colorless and tasteless it is a difficult gas to detect, to differentiate from other gases, and to measure its relative concentration in a given volume of air. Some of the known methods of detecting and measuring the amount of carbon monoxide in air, actually measure the amount of some other gas such as carbon dioxide which is more easily detected and measured. Then the ratio of carbon monoxide to carbon dioxide is calculated for given conditions, which gives a rough approximation and is a suitable detector of carbon monoxide for various purposes. However, for use in stopping the engine of a vehicle when a critical danger level is reached, the detecting mechanism must be relatively precise or the engine may cut out while underway and at a dangerous point, such as when passing another vehicle in the face of oncoming traffic in the passing lane, even though the carbon monoxide level within the passenger compartment had not reached the critical point of becoming a danger to the driver and passengers. Another known method of detecting and measuring the concentration of carbon monoxide in air is by measuring the change in electrical current passing through a silver oxide pellet when carbon monoxide is present. However, this relatively recent discovery as disclosed in U.S. Pat. No. 3,970,431 which issued July 20, 1976, is limited in its application, since the reaction of carbon monoxide with the silver oxide medium is irreversible at higher concentrations, such as above 30 parts per million. Thus, if the concentration of carbon monoxide is above 30 parts per million, the silver oxide pellet or other medium cannot be used again. For the purpose of the present invention, such a prior art method of carbon monoxide detection and measurement would be impractical, not to mention expensive. The level of carbon monoxide concentration in air which approaches the danger level for humans is about 0.14% at the low end and a 0.4% level maintained for about 30 minutes is nearly always fatal. Thus, a detector for the purpose used in the present invention must be operable at some point between 140 and 400 parts per million of carbon monoxide in the atmosphere. Also, a silver oxide pellet or other medium would have to be rigorously protected and shielded from the atmosphere until ready for use. Otherwiseit may be inadvertently exposed to an atmosphere in which a relatively small amount of carbon monoxide is present, e.g. above 30 ppm but below 140 ppm, in which case the silver oxide would become contaminated and irreversibly reacted with the carbon monoxide in the air.
Another method of detecting carbon monoxide known to the prior art is the use of a Wheatstone bridge, comprising balanced resistors in a bridge circuit, one of which is utilized as a sensor to detect the presence of carbon monoxide. The resistor which functions as a sensor is enclosed within a chamber into which continuous samples of the atmosphere are admitted for contact with the resistor. The sensing resistor may be combined with a material such as hopcalite which has properties capable of distinguishing carbon monoxide from hydrogen and other gases which may be present in the atmosphere. Hopcalite is a catalyst which was developed some sixty years ago, about 1918, and which oxidizes carbon monoxide in any air which contacts and passes through the hopcalite material. Thus, when air containing a concentration of carbon monoxide enters the sensing assembly comprising a combination of the sensing resistor and hopcalite, it will oxidize thus raising the temperature of the sensing resistor causing a change in current flow through such resistor. Current flow will increase or decrease depending on whether the resistance material has a positive or negative temperature coefficient of resistance. In either event, the Wheatstone bridge will become unbalanced thus indicating the presence of carbon monoxide. An example of this type of detector is disclosed in U.S. Pat. No. 2,879,663 which issued Mar. 31, 1959.
One problem with the Wheatstone bridge type of detector is its reliability, and its ability to distinguish between carbon monoxide and hydrogen or other gases. Under certain circumstances or conditions, the sensing resistor assembly may oxidize or cause combustion in the presence of gases other than carbon monoxide, or other than a pre-determined level of carbon monoxide concentration which could have catastrophic effects if continued operation of the internal combustion engine of a vehicle were being controlled by such a detecting mechanism.
It is for that reason among others that the present invention includes the use of a plurality of carbon monoxide detectors, at least two of which operate on differing principles so one is a type of "fail-safe" check on the other and both must indicate the pre-determined level of carbon monoxide concentration in the atmosphere within the passenger compartment of the vehicle before initiating the control mechanism which will stop operation of the internal combustion engine, the source of the carbon monoxide gas.